This is a common misinterpretation of something Tim said about multiple castsequences
When you have one, like his example you linked shows, it behaves exactly as expected. It will go 1, 2, 3 all the way to the end and then reset. However, if it gets stuck on any one of those for any reason, the entire line waits until you can somehow continue, like due to a cooldown coming up or generating enough resources.
However, imagine you have two of the exact same sequences in the same macro. We’ll use the one from Tim’s example.
/castsequence Judgement(1), Templar's Verdict(2)
/castsequence Judgement(3), Templar's Verdict(4)
Logically, GSE should go 1, 2, 3, 4, but that’s not what actually happens. What really happens depends on if the macro is set to Priority or Sequential.
If on Priority it will do 1, and then 1 again, and then 1 again. You would have to hit it faster than your GCD if you want it to attempt to beyond 1. But since 1 is in a sequence with 2 it’s now locked because 2 can’t continue due to not having enough resources. So it will actually go 1 (generates 2 HP), 3 (because it can’t do 2, generates 2), 2(spends 3), 1 (because both spells have been used, generates 2), 2 (spends 3, now at 0), and repeat.
Sequential will do the same thing but will try to remember where you left off. 1, 2, 3, 4. Using the above castsequence it will go 1, 2(fail), 3, 4 (which also counts as 2), 1, 2, 3, 4 (fail because of not enough HP), 1, and so on.
Both are examples of why you also want to spam at least slightly faster than your GCD.
That’s about it for Castsequences 101. If you want to string in sequences that are completely different from each other, or are different but contain one or two spells in common with each other, that gets much more complicated. That’s why I likened it to having a deck of cards in two piles and you’re shuffling them together in a bridge, where each stack is a castsequence and they shuffle in with each other and loop back depending on common spells.